The present invention relates generally to magnetic proximity detectors, and more particularly to magnetic proximity switches utilizing a dc magnetic flux as the physical property to be detected.
A magnetic proximity sensor using dc or permanent magnetism for the detection of either unmagnetized ferrous targets or permanently magnetized targets has a variety of advantages over other prior art proximity sensors. More specifically, such a dc magnetic proximity sensor can be more easily immunized against the effects of shock and vibration than magetically actuated mechanical switches because this sensor has no moving parts. Moreover, in contrast to devices using optical windows, the operation of this dc field sensor is unimpaired by dirt, paint, or grease. Additionally, unlike devices utilizing alternating fields, a permanent magnetic sensor can be made impervious to all but very low frequency electromagnetic radiation by using conductive shielding. Moreover, since the source of the field for this device may be a permanent magnet, it can be made very strong as compared to an ac source, without the need to generate it electronically from a power source. Finally, it should be noted that a dc magnetic field sensing device is sensitive to the sense as well as the amplitude of the field. In this regard, the direction of the magnetic vector field of a magnetized target is determined entirely by geometry. Consequently, this sensor can be used to indicate sharply defined positions along a target's line of travel where the field component sensed by the device reverses direction.
However, dc magnetic field proximity sensors in general, and sensors of the type defined in patent application Ser. No. 398,500, filed July 15, 1982, by Frank Heemstra, entitled "DC Magnetic Field Proximity Sensor", now Pat. No. 4,488,113, in particular, have their exact range fixed by a given field threshold level which, in turn, is determined by the strength of the internal magnet used therein, the field strength of the target if a magnetized target is used, and the sensitivity of the detector. For uniformity of results, these quantities must be specified and kept within certain tolerances, thereby limiting the operation of the device.